It could be a RI, the mouth should be almost white, not very pink, is that how it looks? Do you notice any white cheesey looking stuff anywhere in the mouth(esp along the gum line and lips)?

It also could just be he had just gotten a drink of water, but there really shouldn't be bubbles in the mouth. Why don't you give him a few hours, then re-check and see if you still see bubbles? If so, it might be a beginning RI and you may want to take him to a vet.

Hope this helped.